The invention relates to a measuring head for an apparatus to determine photographic data having a light source, an optical projection means to illuminate a measuring spot on a measuring surface, a spherical annular mirror, a flat annular mirror applied to a surface of a glass ring body and a measuring means.
A measuring head of this type having an optical projection means to illuminate a measuring surface and optical measuring means for measuring the light reflected is known from U.S. Pat No. 4,078,858 and comprises an incandescent lamp, which in order to produce a measuring spot, is associated with a collimator lens and a deviation mirror, together with a further collecting lens. The light reflected by the measuring spot at an angle of 45.degree. arrives in the optical measuring means, which comprises a spherical annual mirror and a flat spherical mirror.
U.S. Pat. No. 3,982,824 teaches a mirror system for electromagnetic radiation or acoustic radiation, in which in order to avoid shadowing and chromatic aberration, a primary mirror cooperates with a secondary mirror. The mirror surfaces of the primary mirror are formed by a parabolic segment rotated around the optical axis as the generating line, wherein the parabolic focus describes a circular focal line in a radial plane with respect to the optical axis of the mirror system. The mirror surface of the secondary mirror of the mirror system is formed by the rotation of part of an ellipse around the system axis, wherein a first focal point of the ellipse remains on the optical axis, while the second focal point of the ellipse migrates in a circular focal line around the optical axis of the mirror system, when the ellipse is rotated. All axial rays impacting the primary mirror are collected in the known lens system in the focal point of the secondary mirror. Axial rays impacting the secondary mirror directly are reflected in the direction of the optical axis of the system and do not arrive in the focal point of the ellipse. Axial beams emanating from a point shaped source of radiation at a large distance from the known mirror system, are focused perfectly, so that the mirror system produces an error free image of such beams only.
A reflector layout to concentrate the radiation emanating from a rod shaped flash lamp on a rod extending parallel to the flash lamp and consisting of a laser material, is described in GB-A No. 2,181,265. The reflector layout of the optically pumped laser is in the form of a cylinder aligned parallel to the longitudinal axis of the flash lamp and the laser rod, said cylinder having essentially the shape of an ellipse in cross-section, the focal points and focal lines of which, respectively, are passing through the flash lamp and the laser rod. In order to avoid the shadowing occurring in the area of the principal apex of the ellipse, the cross-section of the ellipsoidal cylindrical reflector has a special configuration in the zone of the two principal apexes, in order to capture the light reflected by a normal ellipsoidal mirror at a dead angle and to reflect it to the intersection of the secondary axis of the ellipse with the mirror surface, or to capture it from there. This is obtained when a deviating ellipsoidal shape is provided in the area of the principal apex of the ellipse, the first focal point of which coincides with the longitudinal axis of the flash lamp or the laser rod, respectively, and the second focal point of which is located where the secondary axis of the principal ellipse intersects with the reflector surface. The cross-section of this cylindrical reflector is thus generated by the superposition of a principal ellipse with four secondary ellipses, wherein all of the ellipsoidal surfaces are located in the same plane.
An apparatus to determine the optical density of photographs described in U.S. Pat. No. 3,244,062, comprises a measuring head with an optical projection means, which images the light of an incandescent lamp on the photographic material and an optical measuring means which captures the light re-scattered essentially perpendicularly from the photographic surface by means of a deviating mirror transversely to the optical axis of the optical projection means and exposes the inlet aperture of a photoelectron multiplier with the aid of a lens. The optical projection means makes it possible to expose the measuring spot, at an angle of incidence of essentially 45.degree. on all sides, to the light originating in the incandescent light in the measuring head. The optical projection means correlated with the incandescent lamp contains a spherical annular mirror, which surrounds the incandescent lamp, with the filament of said lamp extending along the optical axis of the spherical annual mirror. The light of the lamp passing from the spherical annular mirror parallel to the optical axis of the optical projection means is focused by means of a simple annular mirror onto the measuring surface. The quality of the optical projection means and the optical measuring device of the known apparatus is adequate for simple density measurements in the determination of optical reflection properties.
From U.S. Pat. No. 4,025,200 another optical projection device for the laser light of a densitometer or spectral photometer is known. The laser light is expanded, by means of a transparent tube aligned transversely to the direction of radiation, along a plane extending at right angles to the axis of the tube. After passing through a diaphragm, with the aid of a cylindrical lens, located transversely to the direction of expansion, the beam is narrowed, in order to produce a scanning beam, the width of which may be adjusted by altering the distance of the cylindrical lens to the object to be scanned.
The configuration of a spectral photometer with a simultaneous evaluation of the reflection spectrum of moving measuring surfaces is disclosed in U.S. Pat. No. 4,076,421. The light of a xenon flash lamp arrives in the inner space of an Ulbricht sphere provided with a coating for diffusely reflecting the light, in which the measuring surface of the sample to be scanned is also located. Through an aperture, the light reflected from the measuring surface arrives at a lens, which focuses it onto a slit. The slit is correlated with a concave reflection grating, which decomposes the light spectrally in a manner such that individual photodetectors of a linear array are exposed to light of different wavelengths. Because of the high light output of the xenon flash lamp, the intensity losses resulting from the diffuse reflection on the inside of the sphere are acceptable.
From FR-A No. 1,527,717 a portable reflectometer is known which comprises a measuring head with an incandescent lamp and an optical projection means, together with an optical measuring means. The light of the incandescent lamp exposes through a filter on the one hand, directly a first photoresistor, and on the other hand, by means of the projection device and the measuring means, a second photoresistor. The two photoresistors are located in a balancing bridge circuit, whereby the intensity of the light reflected by a measuring surface may be evaluated. The filament of the incandescent bulb in the measuring head is located in the focal point of a parabolic mirror, which comprises a recess for the filter to be inserted. The light leaving the parabolic mirrors essentially in the direction of the optical axis, arrives through a conical optical wave guide or collector made of a refracting material, at a coupling disk of a refracting material, the diameter of which is smaller than the diameter of the parabolic mirror and smaller than the opposing end of the conical collector. During measurements, the coupling disk is pressed solidly against the surface of the object to be examined, said object consisting in particular of food or agricultural products. Along its axis, the conical collector has a cylindrical bore in which a cylindrical glass rod is located. On the side facing the coupling disk, the glass rod has a flat frontal surface adhesively bonded to said coupling disk. On its other end, the glass rod has a spherical surface, the associated focal point of which is located on the surface of the refracting coupling disk, which may be pressed onto the measuring surface to be examined. Inside the bore in the conical collector, the second photoresistor is located, spaced apart from the spherical surface of the glass rod. The first photoresistor, which detects the intensity of the output light, is located on the reverse side of the photoresistor exposed to the measuring light. No spectral analysis is possible with this known portable reflectometer, but only an analysis of the reflection, using different replaceable filters.